Pyrolysis processes, such as steam cracking, can be utilized for converting saturated hydrocarbons to higher-value products such as light olefin, e.g., ethylene and propylene. Besides these useful products, hydrocarbon pyrolysis can also produce a significant amount of relatively low-value, high-viscosity products, such as pyrolysis tar. Steam cracker tar (“SCT”) is a pyrolysis tar obtained by steam cracking hydrocarbon.
SCT is a heavy hydrocarbon which contains relatively high molecular weight hydrocarbon molecules. These high-molecular weight molecules are conventionally called tar heavies (“TH”). It is conventional to dilute SCT with lower-molecular weight hydrocarbon in order to make it more useful as a blendstock for hydrocarbon fuels. In part to avoid the cost of added diluent, attempts have been made to upgrade undiluted SCT by catalytically hydroprocessing, e.g., to decrease viscosity and heteroatom content. It has been observed, however, that catalytically hydroprocessing undiluted SCT generally leads to a rapid onset of significant catalyst deactivation, primarily as a result of coke deposits on the hydroprocessing catalyst. The catalyst coking has been attributed to the presence of TH in the SCT and to the secondary oligomerization of radicals generated during the hydroprocessing.
It is conventional to lessen the amount of catalyst coking by (i) combining the SCT with a utility fluid to produce an SCT-utility fluid mixture, (ii) pre-heating the SCT-utility fluid mixture, (iii) hydroprocessing the preheated SCT-utility fluid mixture, (iv) and separating light gases and at least a portion of utility fluid boiling-range hydrocarbon from the hydroprocessed product to produce a raffinate comprising hydroprocessed SCT. The utility fluid comprises solvent having significant aromatics content. Compared to the SCT feed, the hydroprocessed SCT has a lesser kinematic viscosity, contains less sulfur, and has a lower insolubility number (“IN”), resulting in improved compatibility of the hydroprocessed SCT with fuel oil blend-stocks.
One conventional SCT hydroprocessing technique, disclosed in P.C.T. Patent Application Publication No. WO2013/033590 A1, utilizes a molecular hydrogen partial pressure of at least about 1000 psig (about 68 bar). Although it is conventional to construct vessels that are suitable for hydroprocessing at such high pressures, it is desired to lessen hydroprocessing facility cost by operating the hydroprocessing at a lesser molecular hydrogen partial pressure. It is also desired to re-purpose for SCT hydroprocessing existing hydroprocessing facilities that are designed for other forms of hydroprocessing, which operate at lesser molecular hydrogen partial pressure, e.g., ≦600 psig (about 40 bar). It is particularly desired to develop SCT hydroprocessing which can operate at a lesser molecular hydrogen partial pressure without significant degradation in hydroprocessed SCT properties such as viscosity, IN, and sulfur content. Although operating the hydroprocessing at (i) increased molecular hydrogen: SCT mass ratio and/or (ii) decreased temperature, might permit a reduction in molecular hydrogen partial pressure during hydroprocessing, it is desired to avoid the expense resulting from increasing the amount of molecular hydrogen and to avoid product viscosity increases that are observed when operating other forms of hydroprocessing at reduced temperature.